Such a wing must naturally present aerodynamic performance suitable for driving the helicopter to which it is fitted. It must also satisfy ever-more restrictive standards for noise certification. As a result, the shape of a blade is defined nowadays while taking account of both of these aspects, which can sometimes be contradictory.
The dimensional considerations that govern the aerodynamic performance of a blade are set out in particular in patent document FR 2 689 852, which is incorporated herein by reference.
Concerning the noise effects of the shape of a blade, knowledge is much less advanced.
Nevertheless, a multi-zone blade is known that comprises, starting from the axis of the rotary wing and going away therefrom:                an inner zone of reverse taper ratio, i.e. of chord that increases;        a forward-swept zone of chord that initially increases and then decreases; and        a back-swept zone of chord that decreases.        
It is recalled that by convention a zone is said to be swept “forwards” when its leading edge forms a positive angle relative to the axis of the blade, i.e. extends forwards in the direction of rotation of the rotor, and a zone is said to be swept “back” when its leading edge forms a negative angle.
That multi-zone blade presents advantages in reducing noise pollution. The sequence of a forward-swept zone followed by a back-swept zone avoids the leading edge of the blade being parallel to the axes of the vortices generated by the preceding blades, all along the span of the blade. In addition, the taper ratio of the back-swept zone has a favorable influence on noise since:                for given profile, thickness decreases with shortening chord, thereby reducing so-called “thickness” noise; and        the area of the blade is reduced by the taper ratio, thereby also reducing its lift, which reduces so-called “load” noise.        
Nevertheless, such a blade presents limitations dynamically speaking, since it does not present satisfactory locations for centers of special interest, i.e. the aerodynamic focus, the center of gravity, and the elastic center relative to the pitch axis. This results in excessive control forces and in poor overall dynamic behavior of the rotor.
In addition, its aerodynamic performance is not as good as that of a conventional blade having the same area. In particular, its figure of merit in hovering is smaller and the advancing blade stalls prematurely under heavy weight during forward flight.
The reverse taper ratio of the inner zone is advantageous in that it enables the weight of the blade to be reduced. Nevertheless, it is necessary for the root of the blade, i.e. the beginning of the inner zone, to be capable of withstanding the twisting moments that are generated thereat, more particularly because of the presence of the back-swept zone. It is therefore necessary to stiffen the blade by increasing its thickness on coming closer to the axis of the rotary wing. The profile of the blade is thus very complex since its profile must not only taper going away from the axis of the rotary wing, it must also become thicker goes towards said axis.
There is no need to remind the person skilled in the art of the difficulties involved with such complexity.
It also turns out that the taper ratio of the inner zone severely penalizes behavior while hovering, and that has a direct influence on the maximum takeoff weight.